Official WGPU integration

[s-nie]

Based on the discussion in #1555, it looks like providing an official wgpu backend might have its benefits.

This builds on Nils Hasenbanck's egui_wgpu_backend but removes the need for egui_winit_platform and some boilerplate code and instead uses egui's own egui-winit and eframe. This approach worked well in egui 0.17 using the separate epi crate. 0.18 made this approach invalid as epi was moved into eframe.

@hasenbanck Since this is a heavily modified fork from your repository from around egui 0.16, you are still listed as the primary author. I also left the original license files in here, at least for now. Let me know if this is okay for you.

This does not compile right now. But as outlined in #1555, there is some potential for sharing code between the glow and wgpu backend which we can experiment with in this PR.

[hasenbanck]

@s-nie From a license and copyright standpoint this likes fine.

[emilk]

Great! I continued work on this in #1564 (because I don't know how to commit to this branch), so I'm closing this now.